1. Field of the Invention
This invention generally relates to a system utilizing multiple reciprocating pumps for pumping a fluid into a common pressure outlet and more particularly to an apparatus and method for controlling the phase shift between the strokes of the pistons of the pumps to operate the pumps at desired speeds.
2. Background of the Related Art
Large reciprocating pumps are frequently used to pump a fluid from a source into a common pressure outlet or manifold. The reciprocating pumps contain one or more than one cylinders, each having a piston reciprocating therein. The pistons are moved (stroked inside their respective cylinders) by a prime mover coupled to a crank shaft to pump the fluid into the manifold. The speed of a piston and the pressure exerted by it on the fluid being pumped by it varies as it reciprocates within its respective cylinder. Variable speed a.c. motors, d.c. motors, diesel/gasoline engines, hydraulic systems or the like are usually used as prime movers.
In the oil and gas industry, two or more large electric pumps, each usually larger than 750 horse-power capacity, are frequently used to pump a fluid from a source into a common pressure outlet, from where it is discharged into a well while such well is being drilled. The fluid commonly used is a mixture of mud, oil, water and other materials. Such a fluid is generally referred to as the "mud," and the pumps used therefore are referred to as the "mud pumps." The majority of mud pumps currently used in the industry contain three cylinders and cost more than one hundred thousand dollars ($100,000.00) each.
Typically, all mud pumps supply the fluid into a common pressure outlet or manifold or pipe, from where the fluid is discharged into the well. The speed of each pump is separately controlled by controlling the speed of the prime mover. When electric motors are utilized as prime movers, their speeds are adjusted by varying the power to the motors. In a typical multiple mud pump system used in the oil and gas industry, an operator endeavors to set the speed of each pump the same by manually adjusting a separate throttle for each motor, which as a practical matter is rarely achieved. Further, no means exists to control the phase shift between the strokes of the pistons of different pumps.
The difference in the number of strokes of the mud pumps is generally termed as the beat frequency. The presence of a beat frequency and varying phase shift between the strokes of the pistons of the pumps cause the pressure in the manifold to vary significantly during each pump system cycle. The fluctuations in the pressure (the pressure ripples) cause fatigue on the pump seals, pump valves, output side hydraulic system, flexible hoses, and other parts related to the mud pumps, causing such elements to wear out quickly, greatly reducing the working life of the mud pumps and related devices in the output hydraulic system. Any breakdown of a mud pump or the related parts may require shutting down the drilling operation until the broken mud pump or the part is either repaired or replaced. Such a shutdown can be very expensive, especially for off-shore drilling operations because it costs several thousand dollars for each hour of down-time.
In state of the art drilling of oil and gas wells, well logging tools and other equipment are connected to the drill pipe near the drill bit for determining the presence of hydrocarbons, various geological parameters such as pressure, temperature etc. and other information. These tools contain a variety of sensors and electronics to process electrical signals. The electrical signals are usually transmitted up-hole through the drilling mud. Any unpredictable pressure variation in the mud column may introduce noise into the transmitted signal, which may degrade the quality of the signal being transmitted through the mud column or may require the use of additional circuitry or software to process the signals. Thus, the overall reduction of the pressure ripple (pressure fluctuation) and the elimination of the beat frequency allows better transmission of the signals through the mud column.
The present invention addresses the above noted shortcomings of the prior art pump systems and provides a multiple pump system wherein the speed of each pump is controlled by maintaining the desired phase shifts between strokes of the pistons of different mud pumps.